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This change:
- Adds running the GC until the v8 heap size reaches steady state during
  ModuleSystemTest::TearDown. This is necessary to ensure that
  gin::Wrappable objects are freed, as finalizers for JS objects aren't
  run during isolate shut down.
- Removes a racy check that failed under valgrind.

BUG=389016,406487

Review URL: https://codereview.chromium.org/502893002

Cr-Commit-Position: refs/heads/master@{#294779}